Seismic geophysical surveying is used in a variety of applications. For example in the oil and gas sector seismic surveys may be conducted at numerous different stages of well construction and operation. In particular, once well construction has been completed and the wells are operational there may be a desire to perform periodic seismic surveys in order to highlight any significant changes in the condition of the wells and/or the reservoir over time.
Seismic surveys may also be used for assessing reservoirs for the storage of hazardous or unwanted materials, for example in carbon dioxide sequestrations schemes. In these applications there may again be a desire to undertake periodic seismic surveys to monitor the condition of the site over time.
One type of seismic survey is a surface seismic survey, which involves laying an array of sensors, typically a linear array of geophones, along the surface of an area to be surveyed and measuring the response to a seismic stimulus at the surface. Various types of seismic source for producing a seismic stimulus are known, for instance explosives or air guns can be used, but it is common, especially in the oil and gas industry, to use one or more truck-mounted seismic vibrators, often referred to as a Vibroseis™ truck. The seismic vibrator is capable of injecting low frequency vibrations into the earth and can apply a stimulus with a time-varying frequency sweep, i.e. the frequency of the stimulus varies over time.
This technique of surface seismic surveying relies on the reflection of the seismic waves by geophysical features in the ground structure back to the sensor array at the surface. By determining the response of the sensors to the acoustic stimulus, information about the reservoir and/or the borehole can be determined.
Another type of seismic survey, known as vertical seismic profiling (VSP), involves a geophone array being installed down a wellbore and measuring the response to a seismic stimulus at the surface. Again, by determining the response of the sensor to the acoustic stimulus, information about the reservoir and/or the borehole can be determined.
In both surface surveying and VSP, a seismic source stimuli may be moved away from the sensor in a linear or areal pattern. These techniques are known walk-away profiling or 3D profiling.
Recently it has been proposed to use fibre optic distributed acoustic sensors as the sensor array in seismic surveys. Distributed acoustic sensing (DAS) is a known type of sensing where an optical fibre is deployed as a sensing fibre and interrogated with electromagnetic radiation. Radiation which is backscattered from within the optical fibre is detected and analysed to reveal information about acoustic stimuli acting on the optical fibre in different longitudinal sections of the sensing fibre. Thus the DAS sensor effectively acts as a linear sensing array of sensing portions of optical fibre. The length of the sensing portions of fibre is determined by the characteristics of the interrogating radiation and the processing applied to the backscatter signals but typically sensing portions of the order of 10 m or so may be used in some applications and smaller sensing portions for more precise applications. Note as used herein the term acoustic shall mean any type of pressure wave or disturbance that may result in a change of strain on an optical fibre and for the avoidance of doubt the term acoustic be taken to include seismic waves and also ultrasonic waves.
DAS has several potential advantages compared to the use of geophone arrays. Firstly geophone arrays are expensive and thus are typically deployed for an individual survey and then recovered after the survey for use at another location. For surface seismic surveys the geophones are typically mounted on stakes set into the ground at set intervals. DAS however uses a relatively inexpensive optical fibre as the sensing medium. This means that the optical fibre can be deployed in the area to be surveyed, for example by being buried in the desired arrangement (to protect it from surface environmental effects), and then left in situ after the survey is conducted. This is particularly advantageous where periodic surveys of a given location are required as there are only deployment costs associated with deploying the optical fibre the first time. Also for any subsequent surveys the optical fibre, and hence the position of the individual sensing locations, will be in same place as for the previous survey.
Also the expense of geophone arrays means that the number of individual sensing elements in an array is typically limited. With DAS a fibre of length of about 40 km can be used with 10 m long sensing portions to provide 4000 individual sensing portions.
For VSP the use of DAS can allow the whole of a deep well, say 4 km or more, to be surveyed in one shot. With a typical geophone array there is a limit to the number of geophones that are used and thus the geophone array must be relocated within the wellbore to survey different sections. The results from the different sections may then be stitched together but there can be problems in accurately aligning the positions of the geophones at different depths and also, in the time taken to relocate the geophone array, the environmental conditions may have changed. Further, for a production well inserting a geophone array would require a well intervention which temporarily halts production. Thus sometimes a separate observation borehole may be used—but this may be some distance from the wellbore in question it is most wished to survey. DAS can be used with a single optical fibre which can be inserted during well fabrication and which could even be used when the well is producing.
DAS therefore offers several advantages for seismic surveying and has usefully been employed in seismic surveying. The usefulness of the results obtained from the seismic survey is clearly dependent on the how well the response to the seismic stimuli can be determined. It is therefore desirable to improve the processing of the signals received at the DAS sensor.